This invention relates to an emergency lubrication system for supplying oil to bearings and, in particular, to an emergency lubrication system for supplying lubricating oil to bearings for a large rotating shaft such as as a steam turbine rotor. Large rotating shafts such as a steam turbine rotors require that the bearings supporting them be well lubricated at all times during rotation of the shafts. Large steam turbine rotors used in power plants take about 25 to 75 minutes for coastdown to rest. In existing lubrication systems maintaining an oil supply to the bearings of the rotor of a steam turbine is typically dependent upon electrical power. Some lubrication systems utilize a DC backup to maintain the lubrication system operational during coastdown when there is an AC power failure at the plant. The DC system is powered by batteries that are normally maintained in a fully charged condition. Serious problems will result if the lubrication system is not maintained operational during coastdown of the rotor. In the case of the DC backup if that system should fail for any reason and there is a total loss of AC and DC, the bearings supporting the rotor would suffer a catastrophic failure resulting in extended downtime and great expense to the utility to replace the damaged bearings and other associated equipment. Other designs have used a steam powered pump to maintain flow of oil to the bearings during coastdown.
U.S. Pat. No. 4,309,870 issued to Neil A. Guest et al., dated Jan. 12, 1982 discloses a primary pump operatively connected to the rotor of a turbomachine and independently driven by a stand-by pump. U.S. Pat. No. 2,402,467 issued to Thompson, dated June 18, 1946, discloses a system of multiple pumps in which the primary oil pump is turbine driven, and scavenge pumps are electrically driven. In addition, an electrical supply pump is energized during startup and shutdown to maintain bearing oil pressure.
U.S. Pat. No. 2,751,749, issued to Philip P. Newcomb, dated June 26, 1956, discloses an arrangement of lubricant pumps such that after shut down of the power plant a pump will be put into operation if the temperature of a bearing or bearings exceeds a predetermined temperature.
Most steam turbines take about 25 to 75 minutes in coastdown. During part of this time the lubricating fluid to the turbing bearings is provided typically by a centrifugal pump connected to the turbine shaft. The oil reservoir tank for the lubrication system is typically as much as 30 to 40 feet below the level of the turbine bearings. At reduced speeds, the centrifugal pump does not have the ability to move the lubricating fluid up from the reservoir tank to the bearings. Normally, the main pumpejector system provides oil to the bearings. As the turbine coasts down from rated speed, shaft main pump pressure varies as the square of the speed, for example at one-half speed, there will be one-quarter the normal operating pressure. At some point there will be insufficient oil to prime the ejector to overcome the elevation head and the suction-discharge from the centrifugal pump will be lost. At rated speeds, say 3600 rpm or 1800 rpm for nuclear, normal operating rotor speed, at 60 Hz, the pressure in the lubrication system is 10 to 15 lbs./sq.in, for example. Typically, there is a pressure switch included that monitors the pressure at the bearings. An auxiliary pump is often included that extends to the bottom of the oil reservoir or tank. In the event the pressure continues to lower and AC power is lost, another pressure switch activates the DC powered pump. The batteries are designed to drive the DC pump typically for about an hour or more.
As stated previously, if the AC and DC backups fail, there will be no oil supply to the bearings. It takes about 5 seconds to destroy the babbitt lining in the bearings with concomitant destruction of the steam turbine blades, shrouds and other internal parts. This occurrence would normally cost millions of dollars to repair together with loss of power generation and income therefrom.